In the recent automobile industry, the number of front-engine front-drive (FF) vehicles is rapidly increasing for realizing environmentally friendly vehicles (CO2 emission reduction) by the weight reduction and ensuring the living space. With the rise of the FF vehicles, the constant velocity joint (CVJ) essential for the FF vehicles is widely used. The constant velocity joint, especially a plunging type constant velocity joint, in particular, a tripod constant velocity joint (TJ), a double offset constant velocity joint (DOJ) or the like performs complicated rolling/sliding motions at a certain angle during the driving cycle. This causes a sliding resistance in the axial direction. The sliding resistance will lead to vibrations during the idling mode and swaying of the vehicle at the time of starting and accelerating, and become a source of vibrations to generate the beat noise or the booming noise in the interior of a vehicle at particular speeds. There are various improvements in the structure of the constant velocity joint to solve the above-mentioned problems. However, those improvements are restricted in light of the space and the weight of the joint and also from the viewpoint of cost. Therefore, a grease composition excellent in the performance of reducing vibrations is demanded.
In response to such a demand, there is proposed a grease composition for the constant velocity joint, comprising a base oil, a urea thickener, molybdenum disulfide, and at least one selected from the group consisting of calcium salts of oxidized wax, calcium salts of petroleum sulfonic acid, calcium salts of alkyl aromatic sulfonic acid, calcium salts of salicylate, calcium salts of phenate, overbased calcium salts of oxidized wax, overbased calcium salts of petroleum sulfonic acid, overbased calcium salts of alkyl aromatic sulfonic acid, overbased calcium salts of salicylate and overbased calcium salts of phenate (JP Hei 9-194871 A). Another grease composition for the constant velocity joint is proposed where a metal-free sulfur-phosphorus extreme-pressure agent and molybdenum dithiocarbamate are added to the components of the above-mentioned grease composition (JP Hei 9-324189 A). Further, there is proposed a composition for the constant velocity joint, comprising a base oil, diurea thickener, molybdenum disulfide, molybdenum dialkyl dithiocarbamate sulfide, a calcium salt of petroleum sulfonic acid, a sulfur-phosphorus extreme-pressure agent, and a vegetable fat or oil (JP 2006-96949 A). To meet the recent request for more quietness, there is proposed a grease composition for the constant velocity joint, comprising a base oil, diurea thickener, molybdenum dialkyl dithiocarbamate sulfide, zinc sulfonate, a sulfur-phosphorus extreme-pressure agent, and a vegetable fat or oil (JP 2011-37950 A).
In line with the increased engine power and the higher performance of vehicles, more comfortable ride and more quietness are requested, so that the demands for decrease in the vibration resulting from the constant velocity joint and decrease in the vibrations caused with time are increasing. However, the current grease compositions for the constant velocity joint have the problem that the demand for reduction of the vibrations caused with time cannot be satisfied. The grease composition for the constant velocity joint is required to meet the demand for keeping the quietness.